tree-optimization/106722 - avoid invalid pointer association in predcom

When predictive commoning builds a reference for iteration N it
prematurely associates a constant offset into the MEM_REF offset
operand which can be invalid if the base pointer then points
outside of an object which alias-analysis does not consider valid.

	PR tree-optimization/106722
	* tree-predcom.cc (ref_at_iteration): Do not associate the
	constant part of the offset into the MEM_REF offset
	operand, across a non-zero offset.

	* gcc.dg/torture/pr106892.c: New testcase.

2 files changed, 46 insertions, 2 deletions

diff --git a/gcc/testsuite/gcc.dg/torture/pr106892.c b/gcc/testsuite/gcc.dg/torture/pr106892.c
new file mode 100644
index 0000000..73a66a0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr106892.c
@@ -0,0 +1,30 @@
+/* { dg-do run } */
+
+int a, b, c, d, e;
+int f[8];
+static int g() {
+  while (a)
+    a >>= 4;
+  return 0;
+}
+static int h(int i) {
+  if (i >= '0')
+    return i - '0';
+  //__builtin_unreachable ();
+}
+void __attribute__((noipa)) j(int i) {
+  for (b = 2; g() <= 7; b++)
+    if (i) {
+      for (; e <= 7; e++)
+        for (c = 1; c <= 7; c++) {
+          d = h(b + '0');
+          f[-d + 4] ^= 3;
+        }
+      return;
+    }
+}
+int main() {
+  j(1);
+  if (f[2] != 0)
+    __builtin_abort ();
+}
diff --git a/gcc/tree-predcom.cc b/gcc/tree-predcom.cc
index 5d923fb..a6e45e3 100644
--- a/gcc/tree-predcom.cc
+++ b/gcc/tree-predcom.cc
@@ -1771,10 +1771,24 @@ ref_at_iteration (data_reference_p dr, int iter,
 	  ref = TREE_OPERAND (ref, 0);
 	}
     }
-  tree addr = fold_build_pointer_plus (DR_BASE_ADDRESS (dr), off);
+  /* We may not associate the constant offset across the pointer plus
+     expression because that might form a pointer to before the object
+     then.  But for some cases we can retain that to allow tree_could_trap_p
+     to return false - see gcc.dg/tree-ssa/predcom-1.c  */
+  tree addr, alias_ptr;
+  if (integer_zerop  (off))
+    {
+      alias_ptr = fold_convert (reference_alias_ptr_type (ref), coff);
+      addr = DR_BASE_ADDRESS (dr);
+    }
+  else
+    {
+      alias_ptr = build_zero_cst (reference_alias_ptr_type (ref));
+      off = size_binop (PLUS_EXPR, off, coff);
+      addr = fold_build_pointer_plus (DR_BASE_ADDRESS (dr), off);
+    }
   addr = force_gimple_operand_1 (unshare_expr (addr), stmts,
 				 is_gimple_mem_ref_addr, NULL_TREE);
-  tree alias_ptr = fold_convert (reference_alias_ptr_type (ref), coff);
   tree type = build_aligned_type (TREE_TYPE (ref),
 				  get_object_alignment (ref));
   ref = build2 (MEM_REF, type, addr, alias_ptr);